introduce foreign call

compiler/constrain/src/expr.rs

@@ -858,6 +858,52 @@ pub fn constrain_expr(
                 ]),
             )
         }
+        ForeignCall {
+            args,
+            ret_var,
+            foreign_symbol,
+        } => {
+            // This is a modified version of what we do for function calls.
+
+            // The operation's return type
+            let ret_type = Variable(*ret_var);
+
+            // This will be used in the occurs check
+            let mut vars = Vec::with_capacity(1 + args.len());
+
+            vars.push(*ret_var);
+
+            let mut arg_types = Vec::with_capacity(args.len());
+            let mut arg_cons = Vec::with_capacity(args.len());
+
+            let mut add_arg = |index, arg_type: Type, arg| {
+                let reason = Reason::ForeignCallArg {
+                    foreign_symbol: foreign_symbol.clone(),
+                    arg_index: Index::zero_based(index),
+                };
+                let expected_arg = ForReason(reason, arg_type.clone(), Region::zero());
+                let arg_con = constrain_expr(env, Region::zero(), arg, expected_arg);
+
+                arg_types.push(arg_type);
+                arg_cons.push(arg_con);
+            };
+
+            for (index, (arg_var, arg)) in args.iter().enumerate() {
+                vars.push(*arg_var);
+
+                add_arg(index, Variable(*arg_var), arg);
+            }
+
+            let category = Category::ForeignCall;
+
+            exists(
+                vars,
+                And(vec![
+                    And(arg_cons),
+                    Eq(ret_type, expected, category, region),
+                ]),
+            )
+        }
         RuntimeError(_) => {
             // Runtime Errors have no constraints because they're going to crash.
             True